(a) Field of the Invention
The present invention relates to an ink jet recording head capable of controlling the diameter of an ink droplet ejected from the ink jet recording head to record a gray scale image. The present invention also relates to a method for controlling the diameter of an ink droplet in an inkjet recording head.
(b) Description of the Related Art
A drop-on-demand ink jet printer ejects ink droplets from ink nozzles of an ink jet recording head only when the ink droplets are requested. Specifically, the ink droplet is ejected from the ink nozzle by impressing a drive voltage to the piezoelectric element to generate a pressure wave in the ink chamber.
On the other hand, a stemmed ink jet recording head, such as proposed in Patent Publication JP-B-49(1974)-9622 for example, ejects ink droplets having variable diameters onto a recording sheet to thereby print a gray scale image such as for photographic data.
FIG. 1 shows a cross section of a conventional ink jet recording head, described in JP-A-51-37541, wherein a combination of a piezoelectric element 185 and a diaphragm 184 generates a pressure wave in a pressure chamber 182 of the ink jet recording head 180 receiving therein liquid ink. The pressure wave is transferred to a first nozzle 181, where the liquid ink in the ink supply chamber 183 is ejected from a second nozzle 186 due to the pressure wave while forming an ink droplet 188.
FIGS. 2A and 2B show examples of dot patterns formed by the conventional ink jet recording head 180, wherein a single pixel is formed by a matrix of N.times.N dots 151. In FIG. 2A, the gray scale image is represented by the arrangement of a plurality of dots 151 marked in the matrix, with the diameter of the dots 151 being constant. In this configuration, the number L1 of gray scale levels are expressed by: EQU L1=N.sup.2. (1)
A higher resolution and a larger number of gray scale levels, such as for a photographic image, require a larger number (N) of dots 151 for the matrix (or larger matrix size N) in FIG. 2A. The larger matrix size N also requires a higher resolution for the dot itself due to reduction in the resolution for each pixel.
On the other hand, if the dots have variable dot diameters, such as shown in FIG. 2B, the dots by themselves provide gray scale levels. Specifically, assuming that the number of gray scale levels for each dot is n, the number L2 of gray scale levels in FIG. 2B can be expressed by: EQU L2=n.times.N.sup.2 (2)
In the dot pattern of FIG. 2A, wherein n=1 in equation (2) due to the constant diameter of the dots 151 and N=3 for the matrix size, the number L2 of gray scale levels obtained from equation (2)) is L2=9. On the other hand, in the dot pattern of FIG. 2B wherein n=4 in equation (2)) due to the four levels of the variable dot diameters (151a, 151b, 151c and 151d) and N=3, the number L2 of gray scale levels obtained from equation (2) is L2=36, which is far greater compared to FIG. 2A, whereas the resolution for each pixel in FIG. 2B is not degraded. In short, the variable dot diameter pattern shown in FIG. 2B can increase the number of gray scale levels for the dot pattern without raising the dot resolution.
The control of the dot diameter can be achieved by the amount Q of ink for each ink droplet. The amount Q is expressed by: EQU Z.varies..tau..times.v.times.A. (3)
wherein .tau., v and A are wave motion period of the pressure wave generated in the pressure chamber 182, velocity of the ejected ink droplet and the sectional area of the second nozzle 186, respectively. The velocity (v) of the ink droplet and drive voltage V applied to the piezoelectric element 185 have the following relationship: EQU v.varies.V. (4)
FIG. 3 shows exemplified pressure response characteristics of the ink in the pressure chamber 182, wherein the peak pressure of the ink in the pressure chamber 182 changes Pa to Pd based on the applied voltages V.
The velocity v of the ejected ink droplet changes based on the pressure, and thus based on the applied voltage, whereas the wave motion period .tau. does not change. Accordingly, the following relationship: EQU Q.varies.V (5)
can be obtained from relationship (3).
In the ink jet recording head shown in FIG. 1, the voltage V applied to the piezoelectric element 185 is changed so as to control the pressure of ink in the pressure chamber 182, whereby the amount Q of the ink in the ink droplet ejected from the second nozzle 186 is controlled.
It is noted that the change of the velocity v of the ejected ink droplet affects the image quality of the conventional ink jet recording head. This is caused by deviation of the position at which the ink droplet reaches the recording sheet due to the variations of the ratio of the relative velocity between the recording head and the recording sheet to the velocity of the ejected ink droplet.
In addition, when a small ink droplet is ejected, the small ink droplet generally has a lower velocity and tends to stay in the vicinity of the second nozzle, causing stains in the ink jet recording device. This problem may be solved by a recording head proposed in JP-A-51-37541, wherein an air passage 189 is provided outside the pressure chamber 182 and a third nozzle 190 is additionally provided in front of the second nozzle 186, as shown in FIG. 1.
In the illustrated example, an airflow 191 flowing out of the third nozzle 190 at a constant velocity is generated by an air pump or an air accumulator installed outside the ink jet recording head 180. The ink droplets 188 ejected from the second nozzle 186 are lead by the airflow 191, whereby any ink droplet has a velocity equivalent to the velocity of the air flow 191. This proposal may solve the problem as described above. However, the proposed ink jet recording head has larger size, complicated structure and larger weight due to provision of the air passage 189 and the air pump or accumulator.
In an alternative of the above proposal, another ink jet recording head is proposed in JP-A-61-100469, wherein it is noted that the wave motion period of the pressure wave is acoustic and inherent to the pressure chamber.
Specifically, it is noted that the amount Q of the ink in the ejected ink droplet can be controlled based on the natural period .tau. of the ink pressure wave while maintaining the velocity v of the ink droplet at a constant. To obtain different diameters for the ink droplets, a plurality of ink passages having different natural periods are provided in the ink jet recording head, wherein different nozzles eject respective ink droplets having different diameters. The proposed ink jet recording head has, however, drawbacks of increased head size and higher fabrication costs.
Another drop-on-demand ink jet recording head, proposed in JP-A-62-174163, has a configuration wherein one or each of a plurality of piezoelectric elements is attached to the location corresponding to the belly portion between adjacent nodes of one of waves of the natural oscillation modes of the ink in the ink passage. The piezoelectric element thus located is driven to generate a corresponding oscillation mode.
FIG. 4A shows the configuration proposed in JP-A-62-174163 as mentioned above, wherein the piezoelectric element 172 (shown by a dotted line) is located within an ink passage 171 at the location corresponding to the belly portion sandwiched between adjacent nodes of the wave of the tertiary natural oscillation mode, and FIG. 4B shows the wave of the tertiary natural oscillation mode of the ink in the ink passage 171.
The length of the piezoelectric element 172 is designed equal to the length of the portion of the ink passage 171 corresponding to the belly portion between adjacent nodes of the tertiary natural oscillation mode, and the piezoelectric element 172 is located at the belly portion 175 between these adjacent nodes 176 and 177.
The piezoelectric element 172 is driven by a drive voltage having a waveform corresponding to the tertiary natural oscillation mode, to generate a pressure wave having the tertiary oscillation mode in the ink in the ink passage 171. Thus, the pressure wave having a relatively small wavelength can eject a small ink droplet.
A quartic or higher-order natural oscillation mode can be also obtained by attaching a plurality of piezoelectric elements to the locations corresponding to the bellies of the quartic or higher-order natural oscillation mode, and driving the attached piezoelectric elements by a drive voltage having a waveform corresponding to the natural period.
The ink jet recording head thus proposed is generally suited to generate a fundamental oscillation mode and an additional higher-order oscillation mode corresponding to the location of the piezoelectric element or locations of the piezoelectric elements. That is, the proposed recording head can eject only ink droplets having two different diameters corresponding to the fundamental mode and the higher-order mode. Thus, it is not suited to print a gray scale image having a larger number of gray scale levels, such as for photographic image.
Some other recording heads eject a plurality of smaller size ink droplets at a single position, whereby a plurality of gray scale levels are obtained by selecting the number of the ink droplets ejected at the single position. In this configuration, however, a high-speed printing is not achieved due to the iterated ejection of the ink droplets at the single position.